The use of film, especially plastic film, as a dielectric in electric capacitors is known. Such capacitors are distinguished by two types of construction.
Film/foil technology can be regarded as the first construction type. Capacitors of this type contain plastic films as the dielectric and metal foils as electrodes.
Metallised plastic film capacitors are spoken of as the second construction type. They contain a plastic film serving as the dielectric, onto which are vapour-deposited metallically conductive layers as electrodes. Aluminium in a coating thickness of less than 10−7 m (0.1 μm) is preferably used as the metal.
The most important distinguishing feature of these two types of capacitor construction is the behaviour of the capacitor with regard to electric breakdown. This refers to the phenomenon whereby the—in principle insulating—dielectric can be punctured by the electric charges, leading to a short-circuit. Inevitable weak points in the dielectric can be cited as the reason for the occurrence of a puncture. In the first construction type this short-circuit leads to the failure of the capacitor.
The second construction type is described as “self-healing”. In the event of an electric breakdown through the dielectric the vapour-deposited metal coatings in the vicinity of the puncture site vaporise as a result of the high arc temperature. The cause of the short-circuit is effectively burned away. If an electric breakdown occurs, the dielectric forms a highly compressed plasma, which pushes the layers of the dielectric apart. Insulating halos are formed around the puncture. The capacitor remains operational.
Self-healing capacitors are produced from polyethylene terephthalate (PET), poly-propylene (PP), polycarbonate (PC) and polyphenylene sulfide (PPS) films. All of the cited films are partially crystalline plastics.
The disadvantage of partially crystalline plastic is the elevated cost of manufacture. The partially crystalline plastics that can be used to produce films for use as a dielectric in capacitors (with self-healing properties) must display a degree of crystallinity of ≧10% in order for self-healing to be observed.
The production of films from amorphous plastic is more favourable. However, self-healing properties have not hitherto been observed in capacitors having a film made from amorphous plastic as the dielectric.
Capacitors are also distinguished by the way in which they are mounted on a printed circuit board. A distinction is made here between capacitors for push-through mounting and capacitors for SMD (surface-mounted device) applications. Capacitors for push-through mounting are fitted with leads. The capacitors are placed on the top of the printed circuit board. The leads extend through the printed circuit board to the printed conductor side, where they are soldered to the printed conductors.
By contrast, SMD components are attached and soldered directly to the printed conductor side of the board. They are therefore exposed to the soldering heat for a few seconds, which places particular demands on the heat resistance of the dielectric film.
The production of polycarbonate films by casting a polycarbonate solution prepared with a suitable solvent onto a flat substrate, with subsequent evaporation of the solvent and lifting of the resultant film from the substrate, is known according to EP-B1 0 359 953. The film produced in this way can then be stretched monoaxially or biaxially. The use of polycarbonates as electrical insulating materials for electric conductors is likewise known from this patent specification.
In another known prior art Makrofol KG® is used as a plastic film for use as a dielectric in capacitors. This is a partially crystalline, longitudinally stretched cast film made from high-molecular polycarbonate, consisting of bisphenol A homopolycarbonate, with a thickness of 2·10−6 m to 6·10−5 m (2 to 60 μm). Metallised film capacitors with a dielectric made from Makrofol KG® display self-healing properties if a coating thickness of no more than 1.2·10−5 m (12 μm) is maintained.
Dielectrics made from plastics such as Makrofol KG® are worthy of improvement. The production of film from Makrofol KG® is complicated, since after being dissolved it has to undergo a complex filtration process. Furthermore, the temperature limits for long-term stress and heat resistance also cause problems. In continuous use capacitors with a metallised plastic film made from Makrofol KG® are operated in temperature ranges of up to 120° C. This limits their use in hot engine compartments, for example. The heat resistance limit of 140° C. causes problems with the hot soldering of SMD capacitors.
The object of the invention is therefore to provide a film as a dielectric for use in capacitors with self-healing properties, with which it is possible for the capacitors to be used in an elevated temperature range and to be fabricated by the SMD process. The film should be reasonably economical to produce.